An exhaust gas of an internal combustion engine such as an automobile which uses gasoline for fuel contains hazardous components such as hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx). It is necessary to simultaneously purify and exhaust each of the hazardous components using an oxidation-reduction reaction. For example, it is necessary to purify in such a manner that the hydrocarbon (HC) is converted into water and carbon dioxide by oxidation; the carbon monoxide (CO) is converted into the carbon dioxide by oxidation; and the nitrogen oxide (NOx) is converted into nitrogen by reduction.
As a catalyst (hereinafter, referred to as an “exhaust gas purifying catalyst”) adapted to treat these exhaust gases from the internal combustion engine, three way catalysts (TWC) capable of oxidizing and reducing CO, HC, and NOx have been used.
These kinds of the three way catalysts are known, in which a precious metal is supported on a refractory oxide porous material such as an alumina porous material having a high-specific surface area and the precious metal is supported on a substrate, for example, a monolithic substrate made of a refractory ceramic or metallic honeycomb structure or on refractory particles.
Since a binding force between the noble metal as a catalytically active component and the substrate is not so strong, it is difficult to sufficiently secure a supported amount even when the precious metal is intended to be directly supported on the substrate. In order to support the sufficient amount of catalytically active component on a surface of the substrate, therefore, the precious metal has been supported on a catalyst carrier having a high specific surface area.
A porous material consisting of silica or alumina and a refractory inorganic oxide such as a titania compound is conventionally known as the catalyst carrier. Further, in recent years, a catalyst carrier consisting of an apatite-type composite oxide has received the attention as a catalyst carrier which has excellent heat resistance and can prevent sintering of metal catalyst particles supported thereon.
As the catalyst carrier consisting of the apatite-type composite oxide, for example, Patent Document 1 (JP 7-24323 A) discloses a catalyst carrier consisting of an apatite-type compound expressed by a general formula: M10.(ZO4)6.X2 (where, some of or all of M contain 0.5 to 10 wt % of one or two or more transition metals selected from Group 1B and/or Group 8 of the Periodic Table and preferably one or two or more transition metals selected from copper, cobalt, nickel, and/or iron, Z represents a cation of 3- to 7-valent, and X represents an anion of 1- to 3-valent).
As a catalyst which achieves exhaust gas purification effect even in a relatively low temperature state and achieves purification performance as a three way catalyst even in a high temperature range, Patent Document 2 (JP 2007-144412 A) discloses an exhaust gas purifying catalyst consisting of a composite oxide expressed by (Laa−xMx) (Si6−yNy)O27−z, and a precious metal component that exists in the composite oxide as a solid solution or is supported on the composite oxide, which exhibits high low-temperature activity and excellent heat resistance, and which can obtain stable exhaust gas purification performance.
Patent Document 3 (JP 2011-16124 A) discloses an exhaust gas purifying catalyst consisting of a complex oxide expressed by a general formula: (Aa−w−xMwM′x) (Si6−yNy)O27−z (in the formula, A is a cation of at least one of La and Pr; M is a cation of at least one of Ba, Ca, and Sr; M′ is a cation of at least one of Nd, Y, Al, Pr, Ce, Sr, Li, and Ca; N is a cation of at least one of Fe, Cu, and Al, and the following are satisfied: 6≦a≦10, 0<w<5, 0≦x<5, 0<w+x≦5, 0≦y≦3, 0≦z≦3, A ≠M′, where x≠0 when A is a cation of La) and a precious metal component that exists in the composite oxide as a solid solution or is supported on the composite oxide.